1. Field of the Invention
The present invention concerns a synthesis method by plasma chemical vapor deposition for synthesizing thin films or depositing powders on the surface of a substrate by utilizing plasmas and, more in particular, it relates to a synthesis method by plasma chemical vapor deposition capable of stabilizing plasmas and synthesizing thin films such as of aluminum nitride with preferred oriented crystals.
2. Description of the Prior Art
Among ceramic materials, aluminum nitride (hereinafter simply referred to as AlN) has been noted as a stable insulation material of high electric resistance. There have been expected application uses, for example, as semiconductor insulation films for polycrystalline AlN, as SAW devices for single crystal AlN and as diffusion masks for amorphous AlN.
As the method of synthesizing AlN, while there may be considered reactive sputtering method, ion plating method, etc., a method of utilizing chemical vapor deposition (hereinafter simply referred to as CVD) may also be considered. Japanese Patent Application Sho 63-29678 which is a patent application prior to the present application discloses an example of a method for synthesized AlN by CVD. According to the method, aluminum, bromide (AlBr.sub.3) is used as a source, which is supplied together with a hydrogen gas (H.sub.2) into a reaction chamber. Further, a nitrogen gas (N.sub.2) is supplied as an assist gas. For electrically discharging the gaseous mixture into plasmas, microwaves are used, for instance. When the gaseous mixture is subjected to electric discharge into plasmas, radical bondings comprising a combination, of, Al and Br, N and H, Al and N, or Al, Br and H are present. Further, the radical bondings in the combination described above are brought into a resonance state under excitation by the microwaves and under the effect of the microwaves to the specific dielectric constant and the angle of dielectric loss, and are separated into respective elements, in which Al, N, etc. are maintained in a thermodynamically equilibrium state. Then, they are reacted at the surface of a substrate to form thin AlN films.
However, in the method of synthesizing AlN by the plasma CVD described above, the reaction states and materials to be synthesized remarkably vary depending on various conditions, and stable synthesis has been possible only under the conditions within a narrow restricted range. Further, even if a stable synthesis can be obtained occasionally under a certain conditions, it is difficult to reproduce. Further, use of argon gas (Ar) for the synthesis of material by means of plasma CVD may also be considered. However, the argon gas has hitherto been considered as a means for establishing plasmas. In the case of using argon, the argon gas is usually supplied at first to a reaction chamber and then electrically discharged into plasmas, for example, by means of microwaves. Then, aluminum bromide or nitrogen gas is supplied as less decomposable material and, subsequently, the supply of the argon gas is interrupted. However, even with this method, no stable reactions can be obtained and reproducibility is poor.
Therefore, for improving the deposition rate, crystallinity, orientation factor, film quality, etc., the prior art, has mainly employed increasing substrate temperature, increasing the amount of starting material supplied or increasing the degree of vacuum reaction chamber. However, in a case of increasing substrate temperature, the temperature range capable of conducting satisfactory synthesis, for example of a film of material with a satisfactory orientation factor is extremely narrow, and setting the substrate temperature is difficult. Further, if the amount of the starting material supplied is increased, sputtering is likely to be caused, hindering the improvement in the deposition rate. Further, of increasing of the degree of leads to vacuum a reduction in the orientation factor in the AlN film formed.
The present invention has been achieved in order to overcome the foregoing problem in the prior art and it is an object thereof to provide a synthesis method by plasma CVD capable of stable synthesis within a wide range of conditions and providing a satisfactory orientation factor for the synthesized material.